1. Field of the Invention
The present invention relates to a method for manufacturing a substrate for an integrated circuit ("IC") card in which an IC module is embedded, and, in particular, to a method for manufacturing a substrate for an IC card formed by injection molding the IC card substrate to provide a recessed section for housing the IC module.
2. Description of the Relevant Art
In recent years, many types of membership cards, including cash cards and credit cards, and various types of other cards such as hospital consultation cards have proliferated. Such cards are progressively becoming an essential part of our daily lives. Currently, these cards are mainly magnetic cards, but there are limitations to the expansion of the functions of these magnetic cards because they contain small-volume memories. In addition, as is commonly known, cards on which a CPU, memory chip and the like are mounted have been appearing as multifunctional cards with a security function.
In a process for manufacturing conventional IC cards, an embedding hole for an IC module is provided by stamping, end-milling, drilling, and the like, at a specified position on a laminated PVC (polyvinyl chloride) or a PVCA (polyvinyl chloride acetate) sheet, and giving it a depth suitable for receiving an IC module containing a built-in IC device. Thereafter a laminating sheet for supporting the IC module is temporarily attached by laminating, the IC module is embedded in the embedding hole, and lastly a press-laminating method is applied on the sheet, or a spot-facing process is applied on a sheet of PVC or the like by end-milling so as to form an IC module housing recessed section in the sheet. However, with this process, a large volume of cuttings is produced, not only making it difficult to produce a high quality IC card, but also resulting in high costs. When an IC card produced from PVC or the like in this manner is burned, a noxious gas is produced, causing pollution and thus giving rise to various environmental problems and the like.
Accordingly, instead of using PVC, injection molding is carried out using low-polluting ABS (acrylnitrile/butadiene/styrene copolymerized resin). Such technology for producing IC cards at a low price is disclosed in Japanese Laid Open Patent Applications 64-56595 and 1-159295.
A summary of the injection molding technology disclosed in the above-mentioned Japanese Laid Open Patent Applications 64-56595 and 1-159295 is as follows. A substrate forming mold is made up of an upper mold and a lower mold, and a cavity of the same shape as an IC card substrate is formed inside the substrate forming mold. In addition, a projection for forming an IC module housing recessed section for the IC card substrate is provided on the lower mold. A gate for injecting the synthetic resin is also provided in the lower mold.
This injection molding technology will now be explained with reference to FIGS. 1 to 5. FIG. 1 is a sectional view of the principal parts showing the state where a synthetic resin has been injected into an IC card substrate forming mold. An IC card substrate forming mold 20 comprises an upper mold 21 and a lower mold 22. A cavity 23 of the same shape as an IC card substrate is formed at a so-called parting line (PL) 35 where the upper mold 21 and the lower mold 22 are positioned in opposition. In addition, a stepped projection 24 is provided for forming an IC module housing recessed section. A gate 25 for injecting synthetic resin is also positioned on the lower mold 22. A synthetic resin 26 such as ABS or the like is passed through the gate 25 from a liner 27 to fill the cavity 23, and is allowed to stand for a specified cooling period to form an IC card substrate as shown in FIG. 2.
However, the space (later-discussed residual body section 33) between the stepped projection 24 and the upper mold 21 is extremely narrow, and obstructs the flow of the synthetic resin 26. Generally, in injection molding it is practical to form a large number of units simultaneously. FIG. 2 is a plan view showing the formation of a large number of IC card substrates by injection molding. An IC module housing recessed section 29 in an IC card substrate 28 comprises a first stage substrate housing recessed section 30 and a second stage IC chip housing recessed section 31. In commonly known thin IC card substrates, the residual body section 33 of the IC chip housing recessed section 31 is extremely thin, and it is difficult to make each distance between a pot 32 and each of the IC module housing recessed sections 29 uniform when forming a large number of units of the IC card substrate 28.
In addition, as shown in FIG. 2 and FIG. 4, the above-described gate 25 is provided at a position on the short side of the IC card substrate 28; therefore the molecular orientation of the filled synthetic resin 26 is in the direction indicated by the arrow A.
However, the following types of problems occur in this conventional technology. The body thickness of the residual body section 33 of the IC module housing recessed section 29 of the IC card substrate 28 shown in FIGS. 1 to 5 is extremely thin, as described above. Accordingly, even in the case where one IC card substrate 28 is formed, the synthetic resin 26 is filled into the cavity 23 of the IC card substrate forming mold 20, passing through the gate 25 via the runner 27 from the pot 32, and the stepped projection 24 becomes an obstacle preventing the flow of the synthetic resin 26 because the spacing at the upper mold 21 is extremely narrow, and it is difficult for all of the synthetic resin 26 to flow into the residual body section 33. The filling of the resin is therefore unsatisfactory. FIG. 3 is a partially enlarged sectional view showing the synthetic resin 26 adjacent to the IC module housing recessed section 29 of the IC card substrate 28 in the mobile state. As shown in FIG. 3, the synthetic resin 26 filled by the application of high pressure finally flows into the residual body section 33, and an opposing section 34 of the resin, a so-called weld portion, is created so that the residual body section 33 is weakened. Even when the synthetic resin 26 temporarily flows as far as the residual body section 33, the shrinkage of the synthetic resin 26 changes as a result of the difference in body thickness so that the residual body section 33 of the formed IC card substrate 28 has a tendency to swell. As shown in FIG. 2, when a large number of IC card substrates are formed, as outlined above, the timing of the flow to the residual body section 33 of the IC module housing recessed section positioned in the respective IC card substrates 28 differs slightly for each cavity 23. It is therefore difficult for the synthetic resin 26 to flow uniformly up to the residual body section 33 of each IC card substrate 28. Even when the synthetic resin is filled using temporary application of an excessively high pressure, there is the problem that flashes appear on the PL surface 35. Furthermore, as shown in FIG. 4 and FIG. 5, the gate 25 is positioned on the short side of the IC card substrate 28; therefore the molecular orientation of the filled synthetic resin 26 is in the direction indicated by the arrow A, and there are various problems such as longitudinal breaking in the IC card bending tests set by the ISO.